The present invention relates to a TFT (thin film transistor), a producing method therefor and a liquid crystal display using a TFT array substrate.
Generally, matrix type liquid crystal displays are so structured that a display material, such as a liquid crystal, is interposed between a TFT array substrate, on which the TFT formed of a semiconductor thin film is formed, and an opposite substrate, and a voltage is applied to the display material selectively for each pixel. On the opposite substrate, a counter electrode, color filters, black matrixes and others are formed. In the following, this structured-LCD (liquid crystal display) using the TFT array substrate is referred to as TFT-LCD.
In the TFT array substrate, at least the TFT comprising a gate electrode; a source electrode; a drain electrode; and a semiconductor thin film and a pixel electrode are arranged for each element in an array form on an insulating substrate made of glass and the like, and when necessary, an alignment film, a storage capacitance and others are provided. Between the pixels of each pair, a plurality of signal conductors, such as gate wiring and source wiring, are arranged in parallel to each other, to form a display area. Further, at the outside of the display area, an input terminal and a drive circuit for driving the TFT are arranged in association with each of the signal conductors.
To fabricate the LCD using this TFT array substrate, the TFT, a gate electrode and a source wiring that are simply called a gate, a source electrode and a source wiring that are simply called a source, and a drain electrode which is sometimes simply called a drain, and other common wiring are formed on a glass substrate into an array form to produce the active area, and also the input terminal, extra wiring, the drive circuit, are arranged around the active area. To allow their own functions to develop, a conductive thin film (hereinafter referred to as a conductive film) and an insulating thin film (hereinafter referred to as an insulating film) are then arranged, as necessary. Also, the counter electrode is formed on the opposite substrate, together with the color filters and the black matrixes.
The TFT array substrate and the counter substrate, after fabricated, are put into a state of being spaced from each other with a predetermined distance so that a liquid crystal material can be filled in a space between them and then are bonded together at their marginal portions. Thereafter, the liquid crystal material is filled in the space between the two substrates to thereby produce the LCD.
Various kinds of semiconductor devices are formed on the TFT array substrate and the opposite substrate for use in LCD by using technique of thin-film. In those semiconductor devices, there are formed the semiconductor film, the insulating film and the conductive film, and further a contact hole extending through an interlayer insulating film and the semiconductor film (hereinafter referred to as semiconductor film) is formed, such that an electrical connection between the layers is allowed.
As the TFT-LCD increases in size and high definition, pure aluminum (Al) or Al alloy material of low electrical resistance having Al as a major component is being desired to be used for the gate wiring and the source/drain wiring, in order to prevent a signal delay, from the points of its characteristics and process. However, when the second electrodes, made of indium tin oxide (ITO) and the like, forming transparent pixel electrodes are brought into contact with the first electrodes made of pure Al or Al alloy, the contact resistivity becomes so high as 1E10 to 1E12 .OMEGA. that good contact characteristics cannot be obtained.
Thus, it was substantially impossible to realize a TFT array substrate that enables the first electrodes, made of pure Al or Al alloy, and the second electrodes, formed of a transparent conductive film of ITO, to be brought into direct contact (connection) with each other through the contact hole opening in the insulating film.
For solving this problem and providing improved contact, the prior art proposed an approach that the first electrodes were formed into a two-layer structure in which chrome(Cr), titanium(Ti), molybdenum(Mo), copper(Cu), nickel(Ni), or the like is deposited on the pure Al or the Al alloy, as disclosed by, for example, Japanese Unexamined Patent Publication No. 253342/1992, Japanese Unexamined Patent Publication No. 305627/1992 and Japanese Unexamined Patent Publication No. 18058/1996.
As aforementioned, in the conventional producing method, the contact resistivity between the second electrodes made of ITO and the like and the first electrodes made of pure Al or Al alloy becomes so high as 1E10 to 1E12 .OMEGA. that good contact resistivity cannot be obtained. Also, in another conventional method in which the first electrodes are formed into a two-layer structure of different materials in order to obtain an improved contact, it is impossible to perform simultaneous etching by using the same chemical agent, thus requiring two etching processes using two different chemical agents, to cause an increase in process complexity. The object of the present invention is to provide a high-performance TFT which is so structured that a good contact resistivity can be obtained in contacting portions between the second electrodes and the first electrodes and which is formed into such a two-layer structure as to enable the first electrodes to be etched simultaneously by using the same chemical agent so that an Al wiring material of low resistance can be used to reduce manufacturing costs and increase productivity; a producing method therefor; and a liquid crystal display.